1. Field of the Invention
This invention relates to digital video tape recorders (DVTRs).
2. Description of the Prior Art
One previously proposed form of DVTR has, as indicated in FIG. 1 of the accompanying drawings, four rotary recording and reproducing heads A, B, C and D mounted on a rotary head drum 1, the heads A to D being arranged in two pairs A and B, and C and D. An input analog video signal is sampled and the resulting samples are pulse code modulation coded to form digital data for recording by the DVTR in oblique tracks on a magnetic tape 2. For recording, the data are divided into 50-line segments and the samples within each of these segments are equally distributed between the two heads of a pair. For a 625-line 50 Hz (50 fields per second) television system there are assumed to be three-hundred active lines per field, and for a 525-line 60 Hz (60 fields per second) television system there are assumed to be two-hundred-and-fifty active lines per field, so in 625-line operation a field occupies twelve tracks, while in 525-line operation a field occupies ten tracks. This is indicated diagrammatically in FIG. 2 of the accompanying drawings, which shows spatially the oblique tracks designated A, B, C and D to correspond to the heads A, B, C and D that recorded them.
It will be seen from FIG. 2, and possibly seen more clearly from FIGS. 3A and 3B of the accompanying drawings, which show on a time scale which of the heads A to D are recording in 525-line and 625-line operation, respectively, that in 625-line operation, because each field occupies twelve tracks, the first fifty lines of each field are always allocated to the same pair of heads A and B or C and D. In 525-line operation, however, because each field occupies only ten tracks, the first fifty lines of each field are alternately allocated to the pairs of heads A and B, and C and D.
In both 625-line and 525-line operation some problems arise in severe error conditions. Thus if, for example, the head A suffered a head clog causing total drop-out, then in 625-line operation alternate 50-line segments are affected, and this results in a banding effect with three bands per picture. On the other hand, in the event of a similar head clog in 525-line operation, the banding effect changes position from field to field and this results in flicker.